The present invention relates to a process for manufacturing a soil treatment composition, to soil treatment compositions and to the use of such compositions.
The invention relates in particular to a process for manufacturing a soil treatment composition using slag from oxygen-conversion steelworks and to soil treatment compositions based on such slag.
Oxygen-conversion steel making slag is a by-product resulting from the oxygen-refining of pig iron, for example using the Linz-Donawitz (LD) process, in order to obtain a steel resulting from the decarbonization of pig iron. This refining is carried out by the addition of lime to the molten pig iron placed in a converter. The lime allows the impurities to be extracted, by reacting on the latter in order to form the slag. In the present case, a slag refers to an oxygen-conversion steel making slag obtained from a single heat of a converter. The amount of lime added to the converter depends on the desired level of refining, that is to say on the quality of steel desired. The free lime or quicklime that has not reacted appears in the resulting slag. Depending on the amount of lime added, the free lime content in the resulting slag may vary significantly, generally between 1 and 15% by weight, and may exceptionally reach values of 17-18% by weight. After cooling, the slag is crushed into the form of granules or aggregates and is treated during a deironing step so as to recover some of the iron that it contains. This deironing step is carried out before and during the crushing, mainly only little afterwards.
For use in the production of roadways in the form of aggregates, the free lime content of this steel making slag has huge problems, because of the very substantial risk of the nodules of free lime trapped in the slag swelling, resulting in the granules bursting. Generally speaking, the various types of slag, in granule form, coming from different heats are stored together. In this case, the coarse granules, with a size greater than 8 mm, are collected by screening to be used, for example, for the production of forest tracks or in the agricultural fertilization field. This is because the higher the free lime content the more friable the slag. Thus, the coarser granules have the lowest free lime content. The other granules, of less than 8 mm in size, have a free lime content varying from 1 to 15% by weight and are generally rejected without being used.
However, it is possible to predict the free lime content for a given slag, according to the lime added to the converter. Thus, when the slag has less than 5% of free lime, it can be used in aggregate form to produce roadways. From 1 to 2% by weight, the aggregates constitute what are called high-quality aggregates which can be used to produce the surface layer or bituminous mix, on the surface of a roadway. From 2 to 4.5-5% by weight, the slag may be used to produce the intermediate course and the base course of a roadway. Above a free lime content of 5%, it is considered that the slag cannot be used in granule form because of the abovementioned risk of the granules bursting. In order for this slag to be economically utilized, it has therefore been envisioned to limit its free lime content. A first solution consists in injecting silica and oxygen into the converter, after the steel has been discharged, in order to make the free lime react. This solution allows the free lime content to be reduced down to 1 to 2% by weight, but proves to be very expensive. A second solution consists in subjecting the granules to prolonged storage in the open air, for several months. This storage makes it possible to hydrate the granules and thus lower the free lime content down to a threshold of 4.5 to 5% by weight. However, this solution does not allow hydration of the free lime nodules in the core of the granules in order to make them burst.
Moreover, to stabilize and reinforce a soil, especially for the preparation of roadways, it is known to spread lime over the soil in a first step and then treat the soil with cement in a second step. The lime treatment is intended especially to break up the clumps of soil by flocculating the clayey particles. To replace this lime-then-cement treatment, patent application FR 2 733 774 provides treatment products comprising an ungraded oxygen-conversion steel making slag or one with an unselected initial free lime content and blast-furnace slag, which products are preferably ground. In a first step, the soil is mixed with an oxygen-conversion steel making slag. In a second step, the soil is mixed with the blast-furnace slag which acts as hydraulic binder. The steel making slag is used to flocculate the clay particles of the soil to be treated and to activate the blast-furnace slag. However, that document, which does not specify the precise nature of the steel making slag used, requires large amounts of such steel making slag having an unselected free lime content, which are not easily transportable because of their high density, in order to obtain the desired effect.
Thus, the steel making slag having a free lime content of greater than 5% is currently very little used for the aforementioned reasons, such slag representing more than 55% of the total tonnage of steel making slag produced in France. The object of the invention is to provide a solution allowing this oxygen-conversion steel making slag to be used.
For this purpose, the invention provides a process for manufacturing a soil treatment composition, which comprises a step of selecting one or more slags having a predetermined free lime content of greater than or equal to a first threshold value, a step of adding free lime to the slag or slags and a step of grinding the slag or slags, in order to obtain a soil treatment composition having a predetermined free lime content of greater than or equal to a second value.
Advantageously, the selection step consists in selecting one or more oxygen-conversion steel making slags.
According to a first method of implementation, said step of adding free lime consists in adding free lime to an oxygen-conversion steel making slag heat, in the molten state, coming from a converter for smelting the pig iron into steel using a charge of lime and then in cooling said heat in order to obtain a solid overlimed slag having a free lime content of greater than or equal to the second aforementioned value, said grinding step consisting in grinding one or more overlimed slags obtained in the preceding step.
According to a second method of implementation, said step of adding free lime consists in adding free lime to one or more oxygen-conversion steel making slags in the solid state, obtained after cooling one or more slag heats coming from a pig iron-to-steel converter using a charge of lime, said addition step being carried out before or during said grinding step, so as to obtain an intimate mixture of slag(s) and free lime, said intimate mixture forming the treatment composition.
According to one feature, the step of adding free lime is preceded by a step of chemically analyzing the slag or slags in order to determine the necessary amount of free lime to be added in order to attain or exceed the second aforementioned value.
According to another feature, said selection step consists in selecting slag heats in the molten state coming from a pig iron-to-steel converter, according to the charge of lime added to said converter in order to prepare the desired steel.
Advantageously, the process comprises a step of deironing the oxygen-conversion steel making slag(s) in the solid state.
Moreover, the invention provides a first type of soil treatment composition, characterized in that it comprises one or more ground slags having a free lime content of greater than 18% by weight.
The invention provides a second type of soil treatment composition, characterized in that it consists of an intimate mixture of one or more ground slags having a free lime content of greater than or equal to 5% by weight and of free lime, said mixture having a free lime content of greater than 15% by weight.
The invention furthermore provides the use of the aforementioned soil treatment compositions to dry and flocculate a soil.